Coronary–to–Pulmonary Fistulae: What Are They? What Are Their Causes? What Are Their Functional Consequences?

Vascular connections between coronary arteries and extracardial structures ¹ are commonly called fistulae, but few of them feature fistulous flow and few, if any, have any effect upon coronary functioning.

What Should Be Called a “Fistula”?

In a recent review, ¹ I suggested that there is a need to differentiate abnormal or rare connections between 2 vascular structures without fistulous flow (e.g., coronary artery–to–left ventricle, or circumflex artery–to–right coronary artery) from abnormal connections between 2 vascular structures with fistulous flow. A true fistula of the circulatory system is characterized by a clearly ectatic vascular segment that exhibits fistulous flow and connects 2 vascular territories governed by widely variant hemodynamic environments (large pressure differences).

Collateral or aberrant-origin arteries connect 2 neighboring systemic arteries, or they may connect 2 vascular territories at different pressure, by means of a high-resistance tract possessed of a comparatively narrow lumen. These are not fistulae.

By coincidence, this issue of the Texas Heart Institute Journal reports 2 cases of apparent coronary–to–pulmonary fistulae, 1 congenital ² and the other acquired, ³ which gives us an illustrative context for a broad discussion of the definition, causation, and functional consequences of these abnormal vascular connections.

What Is the Case Presented by Cijan?

The case reported by Cijan and colleagues ² in this issue seems to be one of localized pulmonary dysplasia, which is commonly accompanied by an abnormal vascular supply that originates from the thoracic or abdominal aorta. When their origin is through the coronary arteries, similar abnormal vessels have been variously labeled in the literature as coronary fistulae, as coronary–pulmonary collaterals, or (erroneously) as coronary–bronchial fistulae. ¹ Typically, such abnormal vessels are long channels with high vascular resistance, which carry limited blood flow to the lung parenchyma, while they empty into a branch of the pulmonary artery—but never into a bronchial artery!

One cannot agree with the authors' calculation of the amount of shunted blood (or fistulous flow). ² They state that the pulmonary–to–systemic cardiac output ratio is 1.7:1, for a fistulous flow of 1.7 liters per minute (70% of 2.43 L/min), which cannot be expected through the relatively small channels that appear in their angiograms. Such a flow rate, indeed, corresponds to that of the abdominal aorta. The shunt calculations are incorrect in their assumption that the left pulmonary artery has the same oxygen saturation as the main pulmonary artery, as if one were calculating the shunt of a ventricular septal defect. This method of flow calculation cannot be applied in the present case, because the mixing of arterial and venous blood occurred only at a peripheral branch of the left pulmonary artery. The coronary arteries did not exhibit severe ectasia of the segments proximal to the atrial branches involved, nor did they exhibit fistulous flow. Granted, the anomalous vessels connected 2 territories with different hemodynamic conditions (systemic and pulmonary), and the connections were probably congenital in origin. The finding of a normal pulmonary pressure testifies to the high resistance (and consequent pressure drop) that was present in the long aberrant vessels and to the relatively small amount of shunted blood (which one would estimate, on the basis of the angiogram, to be in the range of 50 cc/min). In summary, this case appears to be one of aberrant-origin arteries, in the absence of fistulous flow.

Do “Fistulae” That Originate from the Coronary Arteries Reduce Nutrient Blood Flow in the Coronary Branches?

In the literature and in clinical practice, one encounters frequent mention of the concept of coronary steal, i.e., competitive flow from a common coronary supply towards nutrient coronary and non-coronary branches, or towards fistulous tracts. The theory is that runoff from a comparatively large proximal arterial segment occurs preferentially through a lower-resistance vascular bed (like a fistula), which reduces flow to the higher-resistance nutrient coronary branches. Yet there are difficulties in documenting the actual occurrence of this steal phenomenon: 1st, the congenital nature of the fistulae tends to create a proximal mixed trunk of appropriate size ¹; and 2nd, the hemodynamic balance between the fistulous runoff and the nutrient branches improves when cardiac load is increased (which causes coronary arteriolar vasodilation), to the extent that effort-related angina or ischemia is usually absent. ⁴ If functional ischemia or infarction does occur in a patient with a coronary fistula, this is generally the result either of coronary occlusive disease in the nutrient branches or of fistulous tract degeneration (ectasia, clot, or vascular wall deterioration, with arterial narrowing at the origin of nutrient branches ¹). Steal, by itself, is quite an improbable cause of critical myocardial ischemia in the great majority of cases.

The patient whose case is reported by Cijan and colleagues ² seems to have had quite extensive and diffuse coronary obstructive disease, but 2 obstructions (of the right coronary and the circumflex) are indeed located next to the points at which the respective nutrient vessels arise from the fistulous proximal mixed trunks. Most likely, this patient had an independent aggressive form of atherosclerosis in the presence of increased risk factors, and the fistulae perhaps played a role only in focusing the disease process on the ostia. Ligation of the fistulae at the time of distal bypass surgery was probably not needed in this case, either to prevent coronary insufficiency or to alleviate volume overload to the ventricle; yet it was an obvious completion of the required bypass revascularization procedure. Indeed fistula ligation is frequently recommended without sound justification, even in patients who do not have associated coronary obstructive disease. ⁵

What Are the Causes of Coronary–to–Pulmonary Fistulae?

The case presented by Cijan and colleagues ² confirms previous observations, both in cardiac and extracardiac fistulae, ¹ that coronary fistulae frequently have multiple supplying sources. For example, a coronary fistula to the distal coronary sinus typically has feeding branches from both the right and circumflex coronary arteries. ¹ In the present case, ² a bronchial artery and 2 coronary arteries were feeding the vascular malformation at a dysplastic left pulmonary segment. Because no experimental animal model for coronary fistulae has yet been developed, ^6,7 statements on the causes of such anomalies are totally conjectural. Nonetheless, it is quite likely that localized pulmonary dysplasia, at an early developmental age, causes the production of organizing proteins that are important in determining vascular migration and vasculogenesis from the neighboring vascular beds. ^1,6 The present case ² is quite similar to pulmonary sequestration, a well-known condition often found in association with cardiovascular (e.g., scimitar syndrome) and pulmonary (e.g., lobular dysplasia) defects. In pulmonary sequestration, a small section of the lung is dysplastic (frequently featuring bronchiectasis) and receives an aberrant blood supply from arteries originating from the thoracic or abdominal aorta. ⁵ It is probable that most such pulmonary anomalies have their origin in an anomalous budding of the foregut. ⁵ The dysplastic and dysfunctional lung tissue, if prone to infection, is the real indication for surgery, more than the aberrant arterial supply. In cases of pulmonary sequestration, large fistulae, capable of significant hemodynamic overload or rupture, have never been described in the literature. ⁵

Acquired Disease. In current times, coronary– to–pulmonary fistula is most often acquired, after cardiac surgery. The brief communication from Liu and colleagues, ³ also in this issue, describes left internal mammary–to–pulmonary fistulae observed 2 months after “minimally invasive coronary surgery.” The history is typical: a chance discovery of fistulae at a diagnostic catheterization that was indicated by the occlusion of both the native left anterior descending artery and the implant. In the reported case, the multiple, small fistulae originated from the left internal mammary itself, at the site of the failed distal anastomosis. Most commonly, fistulae in the postoperative cardiac surgery setting arise in the presence of patent or closed coronary bypass conduits (usually mammary); but acquired coronary–pulmonary fistulae have also been described in cases of noncoronary surgery. ¹

Usually, in the reported cases, preoperative selective internal mammary arteriograms are not available to resolve the question of whether the fistula is acquired or congenital. Reports of fistulae originating from coronary arteries (typically from the left anterior descending or diagonal system) to the pulmonary artery in the post cardiac-transplantation setting ¹ provide clear proof that such coronary anomalies can be acquired. Not yet undertaken are prospective and systematic studies on how frequently coronary–to– pulmonary fistulae occur and on the factors associated with such an event (postcardiotomy syndrome? use of the internal mammary artery? opening of the left pleura at surgery?).

Although the pathogenesis of acquired coronary (or mediastinal) to pulmonary artery fistulae is not clear, it seems important to recognize that connections are always established with the arterial (and not venous) pulmonary circulation, a fact that supports a neovascularization mechanism—under the control of specific, organizing proteins ^6,7—that apparently becomes activated in the postoperative state. In regard to the functional consequence of such fistulae, the preponderant evidence is in favor of a trivial (noncritical) steal from the supplying vessel, specifically from the coronary arteries, in spite of a few claims to the contrary, made on the basis of unconvincing individual case reports. ^8–10 The long, narrow, neoformed vessels must indeed have much higher vascular resistance than does the coronary bed with which they must compete. Moreover, during exercise, coronary arterial resistances decrease more than do pulmonary (see the concept of coronary reserve). ⁴

Conclusions

In summary, then, the great majority of coronary– to–pulmonary communications, whether congenital or acquired, must be considered benign, in terms of their effect on coronary physiology. This effect is usually subtle—a nuance, rather than a significant pathologic condition. It is likely that such abnormal vascular paths are caused (either in utero or in life) by local, mediastinal/pulmonary developmental anomalies or injuries, which apparently activate vascular growth and migration out of systemic cardiopericardial arteries, to establish links with the pulmonary arterial circulation.